The invention relates to an electronic device, particularly but not exclusively a display device, comprising thin-film switching elements on a substrate, each switching element including a layer of switching material between first and second electrodes, the layer of switching material lying on the first electrode on the substrate. The invention also relates to the manufacture of such devices.
The invention is particularly, but not exclusively, advantageous in the field of active-matrix display devices, in which the substrate provides one of two parallel support plates of the display device; mutually facing faces of the parallel plates carry picture electrodes of the display device, the switching elements are accommodated between conductor tracks on said one plate and the picture electrodes of said one plate, and an electro-optical display medium is provided between the parallel support plates.
The electro-optical display medium is usually a liquid crystal in practice, but it may alternatively be an electrophoretic suspension or an electrochromic material. The device is called LCD device (for liquid crystal display device) in the former case. The picture electrodes may be arranged in a matrix of pixels which can be activated by means of conductor tracks and the switching elements provided. Such a device, is suitable for displaying television pictures or datagraphics. The switching elements may be PIN diodes, Schottky diodes, or TFTs (thin-film transistors) or alternatively MIMs. The switching material in the case of the PIN and Schottky diodes or TFTs is a semiconductor material, such as, for example, amorphous silicon passivated with hydrogen, .alpha.-Si:H; the switching material in the case of the MIMs may be, for example, tantalum oxide (in combination with tantalum electrodes) or silicon-rich silicon nitride.
A display device of this kind, in which the switching elements are PIN diodes, is known from K. Nieuwesteeg et al., "2.8-inch PAL D.sup.2 R Display for LCD Projector Use", Proceedings of the 13th International Display Research Conference, Aug. 31-Sep. 3, 1993, pp 529-532. The whole contents of this IDRC paper are hereby incorporated herein as reference material. To provide these switching elements, a first electrode is formed for each diode on one of the support plates, a layer of .alpha.-Si:H is provided locally thereon, and a second electrode is formed on top. The two electrodes are strip-shaped and cross one another at right angles. Semiconductor material is present at the crossing point between the electrodes. The electrodes are 4 .mu.m wide, and the diodes are square with a surface area of 16 .mu.m.sup.2.
The process of making the known display device is intricate and expensive. The conductor track patterns are imaged on a layer of photoresist which is provided on a layer of conductive material from which the conductor tracks are to be etched. Expensive photolithographic equipment is necessary, such as a projection printer, for realizing tracks which are 4 .mu.m wide. Such equipment, however, is capable of exposing only a limited surface area in the range of, for example, 100 cm.sup.2 to 500 cm.sup.2 depending on the particular equipment and its cost. A number of exposure steps must be carried out next to one another, joined together with high accuracy, for realizing a picture display device with a larger surface area of, for example, 150 cm.sup.2 or 900 cm.sup.2. In addition, different photoresist masks must be used in this technique, which is known under the name of "stitching".